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Library > Fact Sheets > BMGR Geologic History
BMGR GEOLOGIC HISTORY
Posted 5/1/2009
Printable Fact Sheet
Geologic History
The age of the Earth is estimated at 4.5 billion years, but the oldest rocks in Arizona are about 2 billion years old. The geology of Arizona formed during three geologic eras: Paleozoic, Mesozoic, and Cenozoic.
Paleozoic (240 - 540 million years ago): The North American continent was nearly featureless, and the western half of Arizona alternately elevated and submerged during the Age of Fishes, resulting in marine sedimentary rocks alternated with continental deposits.
Mesozoic (63 - 240 million years ago): Mountains in the central and eastern portions of Arizona were uplifted during the Age of Reptiles, resulting in erosional debris being spread northwestward and westward. The landscape changed to dry desert and sand dunes as the rise of the Sierra Nevada to the west cut off sources of moisture. The North American continent broke off the European continent at the end of the Mesozoic era, drifting westward until it collided with and overrode the Pacific Plate. This resulted in extensive volcanic action and new mountain ranges, including the Rockies.
Cenozoic (Present - 63 million years ago): Continued uplift during the Age of Mammals resulted in plateaus in northern Arizona and other neighboring states, and the deep basins formed by the down dropping between mountain blocks. These were filled over time with the material eroded from the mountains, resulting in the basin and range topography visible today. The Colorado River and other rivers cut through the uplifted materials, excavating deep canyons. By the late Quaternary period the landscape looked much as it is seen today.
The terrain at the Barry M. Goldwater Range is rugged, with a series of northwest-southeast trending mountain ranges separated by broad valleys. These valleys are deeply filled with alluvium (materials eroded from the mountains). This landscape is the result of several mountain building events, separated and followed by the erosional forces of an extremely arid climate.
The sharp-crested sierra mountains visible today are thought to have been produced during the late Tertiary and early Quaternary Basin and Range disturbance. A series of earthquakes during that event simultaneously caused the mountains to thrust upward and the valleys to drop downward along north- to northwest-trending faults, producing a geologic structure commonly referred to as horst and graben (the uplifted mountain range is the horst, the graben is the basement rock in the basin, which becomes overlain with sediments).
Volcanic activity produced bedded mesa-type mountains composed of volcanic ash and lava flows and the basaltic lava flows in the Pinacate and Sentinel Plain volcanic fields. Though tectonic faulting, folding, and volcanic processes are now quiescent, erosion and deposition continue to shape the desert, creating secondary topographic features-- pediments, alluvial fans, bajadas, washes, playas, dunes, and river terraces.
There are 23 mountain ranges on the Goldwater Range, typically rising from 1,000 to 2,000 feet above the valley floors, which range from 100 to 1,800 feet above sea level. Maricopa Peak in the Sand Tank Mountains rises to over 4,000 feet above sea level. The mountains are of two physiographic types: sierras and mesas. Sierras are more predominant on the range, and have a jagged and sharply crested profile. The Mohawk Mountains are an example of a sierra-type mountain. Mesas are blocky and uniform in shape, and relatively flat on top. The Growler Range exemplifies this type of mountain.
Other geologic features of interest on the range are:
Lava cones and flows; alluvial fans and bajadas; washes formed by ephemeral streams; playas, enclosed areas which receive surface waters from ephemeral streams; dunes, composed of semi-stabilized wind-blown sand; river terraces and desert pavements
Suggested reading:
Roadside Geology of Arizona, Halka Chronic, Mountain Press Publishing Company, Missoula MT, 1983.
Source: Luke Air Force Range Natural Resources Management Plan, University of Arizona, 1986.
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